1. Field of the Invention
The present invention relates to a method for the prior monitoring of the detectability of a watermarking signal.
It also relates to a method for determining a partitioning of a set of coefficients, in order to insert a watermarking signal with prior monitoring of the detectability of this watermarking signal.
It also relates to a method of inserting a watermarking signal during the compression of a set of coefficients representing a digital image.
In general terms, the present invention concerns the watermarking of data, representing for example a digital image, with an a priori monitoring of the detectability of the watermarking signal after compression of the data.
More precisely, the concern here is with the application of a watermarking method used at the same time as a compression method optimised in terms of transmission rate/distortion.
2. Description of the Related Art
Watermarking techniques are frequently used for inserting information in a digital image. This information can, for example, make it possible to authenticate copyright on the image.
In practice, this insertion technique consists of modifying certain coefficients representing a digital image, in a manner which is imperceptible but subsequently decodable.
The concern here is with so-called robust insertion methods; that is to say, the inserted watermarking signal can be decoded after various distortions undergone by the image, and, in particular, after compression.
More precisely, this is a case of a statistical watermarking method, based on the slight pseudo-random modification of a certain number of coefficients representing at least part of a digital image in order to insert an information bit. In this type of statistical method, the decoding is effected by applying a statistical test, based on a measurement of correlation, in order to detect the presence of a pseudo-random signal inserted in the digital data.
The statistical test thus carried out makes it possible to associate a measurement of confidence with the decoding.
A method of inserting a robust watermarking signal is known, which makes it possible to guarantee a fixed correct detection probability, as described in the patent application EP 1 043 687 in the name of the Applicant.
The insertion of the watermarking signal is effected in a transformed domain of the digital coefficients (for example, after a discrete wavelet transformation).
The conditions making possible the insertion by modulation of a watermarking while complying with the criteria of detectability and imperceptibility are determined.
For this purpose, a calculation function integrating an additive noise is used. This additive noise, decorrelated from the coefficients, makes it possible to model a distortion applied to the coefficients.
This theoretical distortion is, however, adapted neither to the image being processed nor to the compression technique used for the coding the image.
In particular, the additive noise does not actually model a compression but rather a disturbance during a transmission of the image.